Ambrosia is a vaccine created by multibillionaire Bob Page for the Gray Death. A single dose (20cc) will temporarily cure any individual who uses it, and will provide up to 48 hours of immunity to the virus. After 48 hours, if the individual does not continue using the medication, they will once again be at risk of contracting the Gray Death. Ambrosia is currently the only known vaccine for the Gray Death and is solely produced and monopolized by VersaLife.

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Ambrosia is described as a "non-organic compound containing eutactic components". The Ambrosia vaccine consist of ROM modules which attach themselves to the Gray Death nanites, thus acting as a "hardware-lock". Like the Gray Death, it can only be manufactured using a Universal Constructor.

The beginning of Deus Ex involves the UNATCO agent JC Denton recapturing several containers of Ambrosia which were stolen by the NSF in an attempt to return it to the people. The NSF attempted to ship a sample of Ambrosia to Hong Kong, with the hope that Tracer Tong would be able to reverse-engineer the vaccine and produce it independently.

MJ12 own and control every functioning Universal Constructor on the planet. Considering that a UC is needed to manufacture Ambrosia, they alone decide how much Ambrosia gets manufactured and who it gets distributed to. In this way, MJ12 use the Gray Death and by extension, Ambrosia, as a means to control and manipulate others into doing their bidding.

Based on the description of Ambrosia by Alex Jacobson, it seems that this "vaccine" shares more similarities with antibodies than with real-world vaccines.

Vaccines normally contain a component of a pathogen (virus, bacteria etc.), which is subsequently identified, neutralized, and remembered by the infected individuals' own immune system, granting them long-term immunity. However, it seems that Ambrosia does not work this way. Instead, it latches onto the pathogen (a Gray Death nanite in this case) and prevents it from functioning.

This is very similar to the way antibodies work in animal immune systems. Antibodies attach to a specific part of a pathogen (called an antigen), and either mark it for destruction by other cells in the body, or directly disrupt its ability to function.

Furthermore, real-world vaccines normally give long-term immunization of at least a few years. In Deus Ex, this supposed vaccine offers immunity for a mere 48 hours. Also, vaccines do not offer a "cure" of any sort to someone who has already been infected by a pathogen. This contradicts the ability of Ambrosia to temporarily subside the symptoms of the Gray Death, suggesting that it is not a vaccine at all.